Your top menu type is sure “mainmenu” ?
Go to menu manager and edit your main menu, make sure at his type is “mainmenu”. The title and description doesn’t matter at all for asingment.
After that, go to “Module Manager” and you should find a module named “Menu”. His assigned on “menuload” position. On basic options (right side) select your menu and that’s all.
If you don’t have a module like this, create a new module, select type : Menu.
Make sure it’s published, acces level is public, it’s displayed on all pages, select menu from basic options and as assigned position put “menuload”